The glomerulus is a network of capillaries with many gaps between the capillary endothelial cells. Blood flows under hydrostatic pressure into the capillaries through the afferent arteriole, a branch of the renal artery, and flows out of the glomerulus through the efferent arteriole. The diameter of the afferent arteriole is larger than the diameter of the efferent arteriole. The difference in pressure forces blood plasma through the gaps into the lumen of the Bowman's capsule. This process is called ultrafiltration.